The present invention relates to a water tube boiler, particularly of the type comprising a furnace having at least one burner located in its upper part to generate a generally downwardly directed flame into the furnace, a substantially vertical flue gas stack located close to one side of the hearth and having an inlet for the flue gases at its lower part which is situated in a lower part of the furnace, an outlet for flue gases located at its upper part, a convection tube assembly arranged in the stack for the recovery of heat from flue gases passing through the flue gas stack.
In conventional steam boilers of this type, the convection tube assembly arranged in the flue gas stack generally consists of a plurality of layers of substantially horizontal tubes located one above the other. Occasionally substantially vertical convection tubes have been used. However, the efficiency of the convection part of that boiler has been comparatively low. Both types of known constructions have been found in practice to be highly unfavourable regarding the extent of and difficulty in performing the operations required to repair the convection tube assembly after the appearance of a leak in any tubes.